Category: Locks, Blocks, and Deadlocks

Arun Sirpal shows that the TRUNCATE command needs to take locks like any other data modification command:

The truncate option is fast and efficient but did you know that it takes a certain lock where you could actually be blocked?

What am I talking about? When you issue a truncate it takes a Sch-M lock and it uses this when it is moving the allocation units to the deferred drop queue. So if it takes this lock and you look at the locking compatibility matrix below you will see what can cause a conflict (C).

Arun includes an image which shows what can block what, and also shows us an example.

Dmitri Korotkevitch announces a new tool:

Troubleshooting of the blocking and concurrency issues is, in the nutshells, a simple process. You need to identify the processes involved in blocking conditions or deadlocks and analyze why those processes acquire the locks on the same resources. In majority of cases, you need to analyze queries and their execution plans identifying possible inefficiencies that led to excessive number of locks being acquired.

Collecting this information is not a trivial task. The information is exposed through DMVs (you can download the set of scripts here); however, it requires you to run the queries at time when blocking occurred. Fortunately, SQL Server allows you to capture blocking and deadlock conditions with the blocked process report and deadlock graph, analyzing them later.

There is the caveat though. Neither blocked process report nor deadlock graph provide you execution plans of the statements. Nor do they always include affected statements in the plain text. You may need to query plan cache and other DMVs to get this information and longer you wait lesser is the chance that the information is available. Moreover, SQL Server may generate enormous number of blocked process reports in cases of prolonged blocking and complex blocking chains, which complicates the analysis.

Confirmed to work with SQL Server 2012 and later, but might work on earlier versions as well.  Dmitri has released it to the public, so check it out.

Guy Glantser explains the process around updating data in SQL Server, particularly the benefit of having update locks:

In order to update a row, SQL Server first needs to find that row, and only then it can perform the update. So every UPDATE operation is actually split into two phases – first read, and then write. During the read phase, the resource is locked for read, and then it is converted to a lock for write. This is better than just locking for write all the way from the beginning, because during the read phase, other sessions might also need to read the resource, and there is no reason to block them until we start the write phase. We already know that the SHARED lock is used for read operations (phase 1), and that the EXCLUSIVE lock is used for write operations (phase 2). So what is the UPDATE lock used for?

If we used a SHARED lock for the duration of the read phase, then we might run into a deadlock when multiple sessions run the same UPDATE statement concurrently.

Read on for more details.

Amy Herold has a script to help you find which query is blocking your important query:

It might look complicated but it is actually very simple – query sys.sysprocesses with a cross apply using the sql_handle to get the text of the query, and then an outer apply with the same query again but you are joining to the blocking spid so you can get the text for the query that is doing the blocking. Beyond that, you can filter on various columns and refine your output

Andy Mallon goes one step further and searches for the lead blocker:

When blocking goes bad, it can go really bad. Sometimes it’s because someone (usually, that someone is me) forgets to commit a transaction before going to lunch, and those open locks cause a bunch of blocking. Sometimes a data load runs at a strange time, or an unusual amount of data gets loaded, or a query gets a bad plan and starts running long, or… you get the idea. There are a bunch of reasons this can come up.

The hardest part is that sometimes big blocking chains build up. The session I forgot to commit blocks 5 session. Each of those block 5 sessions. Each of those block 5 sessions… Eventually, I have 8000 sessions waiting on me, and I’m off eating a kale & farro salad. Oops.

The moral of the story is, don’t eat kale and farro salads; that sounds like rabbit food.

Bert Wagner points out that SELECT queries with NOLOCK can still cause blocking to occur:

This is where my understanding of NOLOCK was wrong: while NOLOCKwon’t lock row level data, it will take out a schema stability lock.

A schema stability (Sch-S) lock prevents the structure of a table from changing while the query is executing. All SELECT statements, including those in the read uncommitted/NOLOCK isolation level, take out a Sch-S lock. This makes sense because we wouldn’t want to start reading data from a table and then have the column structure change half way through the data retrieval.

However, this also means there might be some operations that get blocked by a Sch-S lock. For example, any command requesting a schema modification (Sch-M) lock gets blocked in this scenario.

Read on to see which types of commands take schema modification locks, and ways to minimize the pain.